1,2-Dichloroethylene exists in two isomeric forms, cis-1,2-dichloroethylene and trans-1,2-dichloroethylene. The isomers of 1,2-dichloroethylene have distinct chemical and physical properties. In particular, the trans isomer has a lower boiling point, a lower density, a lower viscosity and a lower surface tension than those of the cis isomer. This is why trans-1,2-dichloroethylene (which will be denoted by TDCE) is the preferred isomer in solvent cleaning applications. The two isomers of 1,2-dichloroethylene can be separated by fractional distillation. However, during storage, TDCE is spontaneously converted to the cis isomer, unless it is stabilized, the cis isomer being more stable thermodynamically. U.S. Pat. No. 6,153,575 discloses the use of small amounts of aliphatic aldehyde hydrazones, optionally in combination with an epoxide, for inhibiting the isomerization of TDCE to the cis isomer during storage.
The stability on storage of TDCE is its ability to withstand conversion or decomposition for a period which can range from several weeks to several months in conventional storage containers at a temperature which generally does not reach more than 50° C.
During its use as solvent for the treatment of solid surfaces, TDCE is subject to other attacks, related to the method of operation of the treatment equipment, which can result in its decomposition. The problems generated by the use of TDCE are therefore different from those encountered during storage.
The main sources of attack on TDCE during its use, for example, in a device for degreasing metal components are: air, water, metals and heat. This is because metal degreasing is generally carried out by introducing the components to be degreased into an appropriate device above the liquid level of a tank containing the TDCE maintained at reflux. The TDCE vapours condense on the metal components and the condensate is repeatedly recycled. Under these conditions, the TDCE is subjected to various reactions, such as oxidation by the air, hydrolysis by the water originating from the condensation of atmospheric water vapour, thermal decomposition or catalytic reactions on contact with the metals.
In the literature, various solutions have been provided for stabilizing some halogenated hydrocarbons confronted by the same sources of attack. The addition of various stabilizers is recommended in U.S. Pat. No. 3,043,888 for stabilizing halogenated hydrocarbons comprising from 1 to 3 carbons, more particularly trichloroethylene, during their use in metal degreasing. Some aldehyde hydrazones are particularly effective and can be used in combination with other compounds, such as aromatic compounds comprising a hydroxyl group, amines, organic epoxides, esters of monocarboxylic acids, alcohols or unsaturated compounds. Compounds which are especially effective in inhibiting the decomposition, catalysed by metals, of methylchloroform are described as being 1,3-dioxolane and its alkyl homologues or 1,4-dioxane (see U.S. Pat. Nos. 4,026,956; 3,251,891; 4,418,231 and 2,811,252).
However, the solutions provided in the literature do not include a system for stabilizing a solvent, such as a halogenated hydrocarbon, combining all the characteristics required for a stabilizer, and in particular no complete system for stabilizing with respect to air, water, metals and heat is disclosed in the prior art for TDCE.
First, the stabilizing system preferably has a two-fold action: a preventive action, more particularly in inhibiting side reactions, and a curative action, that is to say trapping in situ an undesirable effect. Subsequently, to facilitate the use of the stabilized solvent, the stabilizing system advantageously has a boiling point similar to that of the solvent, it is advantageously present in the liquid phase and in the vapour phase, it is insoluble or only slightly soluble in water and it preferably does not modify the properties of the solvent. In order not to modify the labelling of the solvent relating to the dangers of use, the stabilizing formulation is advantageously nontoxic. Finally, for economic reasons, the stabilizing formulation is preferably composed of inexpensive industrial products.